1. Field of the Invention
The present invention relates to a rocker arm of a type adapted to be driven by a cam for selectively opening and closing a valve mounted on a cylinder head of the combustion engine.
2. Description of the Related Art
Inexpensive and lightweight rocker arms of this kind are currently manufactured by the use of, for example, a precision casting technique and a press working technique. Of those rocker arms, the rocker arms manufactured by the use of the press working technique to have a generally U-shaped section taken along the line perpendicular to the longitudinal sense thereof have now gained the mainstream in the market. The press-worked rocker arm has a screw-locked-pivot type and a contact-pivot type. The screw-locked-pivot type is illustrated in FIGS. 12A and 12B and is of a design in which a pivot area of a generally elongated arm body 34 is formed with an internally helically threaded hole 42 and a generally elongated pivot member 37 having an externally helically threaded screw shank 37a is threadingly inserted into the threaded hole 42 and is locked in position by means of a lock nut 43 mounted externally on the screw shank 37a. On the other hand, the contact-pivot type which will be described later with reference to FIGS. 10 and 11 is of a design in which a pivot area of a generally elongated arm body is formed with a generally semispherical recess for pivotally receiving therein a correspondingly semispherically shaped abutment end of a pivot projection mounted rigidly on the cylinder head.
In any one of the screw-locked-pivot type and the contact-pivot type, since the pivot area referred to above is an area on which a load is imposed, the diameter of the threaded hole 42 and the size of the abutment end of the pivot projection are determined after strength calculation has been performed. As such, FIG. 13 illustrates the rocker arm of the screw-locked-pivot type in which a connecting wall 36 of the arm body 34 bridging between opposite side walls 35 depending from such connecting wall 36 is chosen to have a wall thickness greater than other portions of the arm body 34 to secure a sufficient strength in the pivot area.
Also, the pivot area of the arm body in any one of the screw-locked-pivot type and the contact-pivot type is defined in a flat outer surface region of the connecting wall 36 bridging between the opposite side walls 35 as best shown in FIG. 12B. In the rocker arm manufactured by the use of the press working technique, an outer surface region (hereinafter referred to as an “outer chamfered corner”) delimited between an outer surface of the connecting wall 36 and an outer surface of each of the opposite side walls 35 is generally rounded to represent a certain radius of curvature R shown in FIG. 12A, which radius of curvature R has hitherto been chosen equal to or greater than the wall thickness. By way of example, even though an inner surface region delimited between an inner surface of the connecting wall 36 and an inner surface of each of the opposite side walls 35 is at right angles relative to each other (hence, the radius of curvature being zero), the radius of curvature R of the outer surface region, that is, the outer chamfered corner is generally chosen to be equal to or greater than 3 mm if the wall thickness is 3 mm.
In view of the foregoing, where in the rocker arm manufactured by the use of the press working technique the pivot area is defined in the connecting wall 36 as hereinabove described, the rocker arm must have a width sufficient to secure the required flat outer surface region in the connecting wall 36 and the required radius of curvature R of the outer chamfered corner. More specifically, in the rocker arm of the screw-locked-pivot type such as shown in FIGS. 12A and 12B, the lock nut 43 firmly threaded onto the screw shank 37a of the pivot member 37 should not loosen under the influence of vibrations due to an automotive vehicle then running and an automotive power plant then in operation and, accordingly, the flat outer surface region of the connecting wall 36 must have a width L2 that is necessarily less than the maximum outer diameter of the lock nut 43. While in the rocker arm manufactured by the use of the precision casting technique it is quite easy to reduce the radius of curvature R of the outer chamfered corner, delimited between the connecting wall 36 and each of the opposite side walls 35, down to a relatively small value, it is not so with the rocker arm manufactured by the use of the press working technique. Specifically, where the required flat outer surface region is to be secured on the connecting wall of the rocker arm manufactured by means of the press working technique, the arm width tends to increase as compared with that in the rocker arm manufactured by means of the precision casting technique, with the consequence that reduction in size and weight of the rocker arm manufactured by means of the press working technique is limited as compared with that manufactured by means of the precision casting technique.